1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more specifically to a signal level conversion circuit for use in a so-called BiMOS semiconductor logic integrated circuit composed of bipolar transistors and MOS transistors which are formed in a mixed manner on the same single chip.
2. Description of related art
BiCMOS semiconductor logic integrated circuits have been composed of CMOS (complementary metal-oxide-semiconductor) transistors and bipolar transistors formed on the same single chip so that the circuits can have both advantages of the CMOS transistors such as a low power dissipation and a high integration density and advantages of the bipolar transistors such as a high operation speed and a high driving power. In fact, the BiCMOS logic circuits have already achieved a gate delay time partially overlapping with that obtained in conventional ECL (emitter coupled logic) logic circuits. Therefore, it has been expected that the ECL logic circuits will be replaced by the BiCMOS circuits in applications or areas where a relatively large gate delay time is permitted.
Considering compatibility with the ECL logic circuits, the BiCMOS logic circuits are required to be driven at the same supply voltage level as that of the existing ECL logic circuits. For example, the BiCMOS logic circuits are required to be capable of operating at a so-called ECL input/output level by introducing a nominal supply voltage of -4.5 V into the chip. On the other hand, taking into consideration the compatibility with TTL (transistor-transistor logic) logic circuits, the BiCMOS logic circuits are preferred to be driven at a so-called TTL input/output level by connecting a nominal supply voltage of +5.0 V to the chip. Therefore, if the ECL circuits and the TTL circuits are combined on the same signal chip, the chip is connected to both a supply voltage of -4.5 V and another supply voltage of +5.0 V. In this case, as a result, there is needed a signal level conversion circuit for converting a signal level between a circuit connected between +5.0 V and the grounds (=0 V) and another circuit connected between -4.5 V and the ground. However, the conventional signal level conversion circuit has been of a differential circuit type, which is relatively large in power consumption and also needs a substantial number of circuit elements. Particularly, the conventional signal level conversion circuit of the differential circuit type requires at least a pair of active circuit elements such as a pair of bipolar transistors, and needs a constant current source.